Dual sourcing magnetic components means qualifying two independent manufacturers for the same inductor, choke or transformer, so that production can continue if one of them is delayed. It is a familiar discipline for semiconductors and connectors, yet magnetics are often left with a single source because they are custom, low in unit value and assumed to be easy to replace. In practice a wound component can be one of the harder parts to substitute at short notice.

This article explains why, and sets out a method for building a second source that behaves the same in your circuit as the first. It is written for purchase managers and design engineers at OEMs of any size. The approach relies on good documentation and measured equivalence, not on any particular supplier or region, and it scales from one critical transformer to a full bill of materials.

Why single-sourced magnetics are a hidden risk

A custom magnetic part exists only in one factory's process. Its drawing may show inductance and dimensions, but the core grade, wire construction, winding sequence, insulation and impregnation often live in the supplier's internal work instruction. If that supplier has a capacity problem, a material shortage or a change of ownership, the buyer discovers that the purchasing specification is not sufficient for anyone else to build the part. Recovery then involves reverse engineering under time pressure.

Catalogue parts carry a different version of the same exposure. A proprietary series can be discontinued or placed on long lead time, and a nominal equivalent from another brand may differ in saturation behaviour, self-resonant frequency or temperature rise. Because magnetics influence efficiency, thermal performance and emissions together, a substitute that passes a bench check can still fail compliance testing. The risk is concentrated in the positions that matter most: main transformers, PFC chokes and EMI filter components.

Own the specification before you need a second source

The foundation of dual sourcing is a specification that describes the part completely and belongs to you. It should state electrical parameters with tolerances and test conditions, the mechanical outline and footprint, the insulation and safety requirements, permitted materials, and the tests applied to every unit. Test conditions matter: inductance measured at a different frequency or signal level gives a different number. The guide on how to read an inductor datasheet shows which conditions sit behind each headline figure.

Decide how much construction detail to fix. A fully prescriptive drawing guarantees similarity but prevents either supplier from using its own preferred core or bobbin. A performance specification gives freedom but allows two parts that differ in second-order behaviour. Most buyers settle between the two: fixing core material class, winding arrangement and critical dimensions, while leaving the supplier to select equivalent materials. Suppliers of custom wound inductors can advise which details affect circuit behaviour and which do not.

• Inductance, tolerance, test frequency and test level
• DC resistance limits and the definition of saturation current
• Turns ratio, leakage inductance and hipot level for transformers
• Core material class, wire type and insulation system
• Outline drawing, pin assignment, marking and packaging

Dual sourcing magnetic components: proving equivalence

Two parts are equivalent when they behave alike in the circuit, not merely when their inductance matches. Compare the samples on an LCR tester across frequency, measure DC resistance, and check saturation by observing inductance against DC bias. For transformers add turns ratio, leakage inductance and interwinding capacitance. Then test in the product: efficiency at several loads, temperature rise at full load in the enclosure, and conducted emissions. Differences in parasitics typically show up in the emissions scan first.

Record the results as an approval report for each source, with the approved sample retained as a physical reference. Agree identical acceptance limits with both manufacturers so that incoming inspection can treat the parts the same way. If your product holds safety or EMC certification, check whether the second source needs to be listed in the certification file; this is a regulatory question for your test house, and it is far easier to settle before a shortage than during one.

Choosing and managing the second supplier

A second source is most useful when it does not share the first one's risks. Consider a different region, different raw material channels and a factory of a different scale. The criteria in the article on how to choose a magnetics manufacturer apply equally to both suppliers: in-house winding and testing, documented quality procedures, traceability and a clear sample process. For buyers looking at South Asia, the overview of magnetics manufacturing in India describes what to evaluate.

Keep both sources active. A supplier that has not built a part for a long time loses familiarity with it, and materials may no longer be on the shelf. Many OEMs split volume unevenly, giving the second source enough regular business to stay current. Share forecast information with both, review test data from each batch, and treat engineering changes as joint events so that the two versions of the part do not drift apart over the product's life.

How a manufacturer can make dual sourcing easier

Suppliers differ in how readily they work to a customer's documentation. A helpful one will build to your drawing, state any deviation it proposes, and report measured values against your limits. NG Electronics is an ISO 9001:2015 certified manufacturer in Pune, India, and works in this way as a power inductor manufacturer and choke coil manufacturer for OEM customers. All parts are wound and tested in-house, every unit is electrically tested, and a test certificate is supplied with orders on request.

Batch records giving traceability to raw material and production date support the comparison of data between sources, and the quality assurance process describes the checks applied. For an existing design, the company can quote from your specification or a reference sample, typically within 24 working hours, and supply a free sample for laboratory approval. A custom toroidal inductor manufacturer able to match an established footprint lets the second source drop onto the current board. Overseas enquiries are welcome.

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